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Accounting for Derivative Instruments and Hedging Activities - Fair Value Within the Derivative Instrument Valuation (Details) - Not Designated as Hedging Instrument - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Derivative [Line Items]    
Derivative assets $ 821 $ 1,170
Commodity contracts 884 1,103
Commodity contracts current    
Derivative [Line Items]    
Derivative assets 560 860
Commodity contracts 499 781
Commodity contracts long-term    
Derivative [Line Items]    
Derivative assets 261 310
Commodity contracts $ 385 $ 322